Wrought aluminum base alloy and method of producing it



Patented June 10, 1941 v WROUGHT ALUMINUM BASE ALLOY AND METHOD OF PRODUCING IT Philip T. Stroup, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.

5 Claims.

' My invention relates to improving aluminum base alloys containing magnesium, zinc, and copper.

Wrought aluminum base alloys containing magnesium and lesser amounts of zinc and copper possess many technically advantageous properties. In making some articles from the cold worked alloys, it is sometimes necessary to heat them to a temperature below the annealing temperature in order to secure the desired degree of workability. The alloys cannot, of course, be heated above the recrystallization temperature and still retain the benefits of the cold working. Since at times it becomes necessary to form articles from heavy sheet or plate that cannot be easily bent at room temperature, it becomes important to shape the material at an elevated temperature, and furthermore to employ sheet or plate which resists the permanent softening effect when heated up to the usual annealing temperature range.

effect caused by raising the temperature of the metal is to be distinguished from the softening produced by recrystallization, since the latter effect persists at room temperature after the metal has been heated, whereas in the former case the softening is dependent upon maintaining the metal at the elevated temperature.

The recrystallization temperature is not only a significant point in the making of articles by the hot forming process, but it also plays a part in making successful brazed joints from cold worked aluminum base alloys. The application of suificient heat to metal parts being joined causes a heating of the metal with the result that recrystallization may occur in the cold worked metal.

The term recrystallization, as understood in the art and as here employed, refers to the process by which new grains are formed from fragments of former grains in plastically deformed metal. As a result of recrystallization,- the strains induced by the cold working are relieved. The temperature at which recrystallization begins varies with the degree of cold work and the rate of heating. The extent of recrystallization depends upon the temperature and the time at temperature. Whenever theconditions do not favor recrystallization, a substantial amount of the effect of cold working is retained by the metal.

An object of my invention is to provide a cold worked aluminum base alloy containing magnesium and lesser amounts of zinc and copper which has desirable workability along with a higher strength and is resistant to permanent softening when heated to the usual annealing temperature range for alloys of' this type. Another object is to enlarge the temperature range The forming of articles in this manner is known as hot forming. The softening Application August 23, 1939, Serial No. 291,545

' This eflect is, of course, modlii above that of the same alloy in- "where the two alloys have be the duration of the thermaltreatm within which the properties of cold worked alloys can be retained to a substantial degree. A further object is to provide a means for raising the recrystallization temperature of cold worked aluminum base alloys containing magnesium, zinc and copper. Still another object is teem-'- vide a material which is free from Liider lines when stretched. p

My invention is predicated on the discovery that the addition of- 0.01 to 1 per cent zirconium to an aluminum base alloy containing from 0.5. to 10 per cent magnesium, 0.25 to 8 per cent zinc, and 0.1 to 3 per cent copper will'ra'ise therecrystallization temperature of the alloy in the cold worked condition to a surprising extent. It

is my preferred practice, however, to use from 0.1 to 0.5 per cent zirconium. I have found, for example, that the recrystallization temperature in many cases may be raised from 200 to 300 F.

mg and est effect being obtainedwhere .ther

exposure to an elevated temperature. Irr'some'. cases recrystallization may be incomplete when, i v the alloys containing zirconium are heatediat merly employed to anneal the metal, namely, 600

to 700 F., and it is thus possible to gainthe advantage of the added workability which is'associated with the use of a higher temperature.

The aluminum base alloys with which this invention is concerned should at all times contain a greater amount ,or magnesium than zinc or copper in order to retain the characteristics of aluminum-magnesium alloys. Cold worked alloys containing from 1m 6 per cent magnesium,

0.25 to 3 per cent zinc, and 0.10 to 1.5 per cent copper are especially benefited by theaddition of zirconium thereto. 7

Another advantage derived from the addition 0! zirconium to the foregoing type of aluminum base alloys is that the appearance of Liider lines is prevented in drawn and formed articles Liider lines are sets of wavy'lines which sometimes appearon the surface of stretched metal but which have no eiIect upon the properties thereof. Because of the irregular appearance-pf the surface, articles exhibiting these Liider lines are usually objectionable in commercial production since the lines can only be removed by a considerable amount of polishing.

Although aluminum-magnesium-zinc-copper alloys such as described hereinabove are useful in and of themselves, it has been found to be advantageous at times to include certain other metals known as hardening elements to enhance some desired properties. stitute this group of hardening elements and the amount of each metal to be used are as follows: manganese, 0.05 to 1.5 per cent; titanium, 0.01 to 0.5 per cent; chromium, 0.05 to 1.5 per cent; vanadium, 0.05 to 1 per cent; cobalt, 0.1 to 1 per cent; and nickel, 0.1 to 2 per cent. The total amount of these elements should not in any case exceed 4 per cent, and preferably less than 2 per cent. The addition of these elements does not destroy the eiiect of the zirconium and in some casesthey may even accentuate the beneficial effect of zirconium in raising the temperature of recrystallization.

The beneficial effect of adding zirconium to aluminum-magnesium-zinc-copper alloys may be better appreciated through reference to the following examples. had the following nominal compositions by by weight:

(A) 1.2 per cent magnesium, 0.5 per cent zinc, 0.15 per cent copper, 0.1 per cent titanium. (B) 1.2 per cent magnesium, 0.5 per cent zinc, 0.15 per cent copper, 0.1 per cent titanium, 0.12 per cent zirconium.

(C) 1.2per cent magnesium, 0.5 per cent zinc, 0.15 per cent copper, 0.1 per cent titanium, 0.22 per cent zirconium.

The balance of the alloys in each case was a commercial grade of aluminum containing about 0.25 per cent of the usual impurities. Ingots of these alloys were hot rolled to slabs 0.25 inch in thickness in the usual manner and annealed, then cold rolled to sheet having a thickness of 0.064 inch, which represented a reduction of '75 per cent from the thickness of the previously annealed material. The cold rolled sheets were heated at 650 F. for 2 hours and cooled to room temperature to observe the softening effect a common annealing practice. Samples from these sheets were tested to determine their strength and elongation. The average values for these properties were as follows:

Tensile Yield Elongastrength strength tion Per cent Three alloys were tested which The metals which con- From these results it can be seen that the zirconium has caused the alloys B and C to resist softening. An X-ray examination of this material showed that recrystalization was complete in alloy A which did not contain zirconium but this element had interfered with recrystallization in the other alloys.

Sheet samples of the above alloys were drawn into small trays to determine whether any Liider lines would appear. It was found that these lines appeared in the case of alloy A but not in trays made from alloys B and C.

The term aluminum base alloy as herein employed refers to those alloys which contain at least 75 per cent aluminum.

I claim:

1. A wrought aluminum base alloy containing from 0.5 to 10 per cent magnesium. 0.25 to 8 per cent zinc, 0.1 to 3 per cent copper, and 0.01 to 1 per cent zirconium, said alloy being characterized by having a higher recrystallization temperature than the same alloy devoid of zirconium where both alloys have received the same amount of work hardening and the same thermal treatment.

2. A wrought aluminum base alloy containing from 0.5 to 10 per cent magnesium, 0.25 to 8 per cent zinc, 0.1 to 3 per cent copper, 0.01 to 1 per cent zirconium, and from 0.01 to 4 per cent of at least one of the hardening elements of the group consisting of manganese 0.05 to 1.5 per cent, titanium 0.01 to 0.5 per cent, chromium 0.05 to 1.5 per cent, vanadium 0.05 to 1 per cent, 0.1 to 1 per cent cobalt, and nickel 0.1 to 2 per cent, the balance being aluminum.

3. A wrought aluminum base alloy containing from 0.5 to 10 per cent magnesium, 0.25 to 8 per cent zinc, 0.1 to 3 per cent copper, and 0.01 to 1 per cent zirconium, the balance being aluminum.

4. A wrought aluminum base alloy containing from 1 to 6 per cent magnesium, 1 to 3 per cent zinc, 0.25 to 1.5 per cent copper, 0.1 to 0.5 per cent zirconium, the balance being aluminum.

5. The method of producing a cold worked aluminum base alloy containing between about 0.5 and 10 per cent magnesium, 0.25 and 8 per cent zinc, and 0.1 and 3 per cent copper and exhibiting improved recrystallization characteristics, comprising the steps, incorporating from about 0.01 to 1 per cent zirconium in the molten alloy at an appropriate stage in its production, casting a body of said alloy, cold working said alloy body, and thereaiter heating the wrought PHILIP T. STROUP. 

